1. Field of the Invention
The present invention relates to a power steering apparatus with an electric motor, inclusive of a type where a hydraulic assist force is generated from a pump driven by an electric motor, and a type where an assist force is generated directly by an electric motor.
2. Description of the Prior Art
In the hydraulic power steering apparatus known heretofore for use in automobiles, it is generally customary that a steering shaft and a pinion shaft are coupled to each other by means of a torsion bar. And in a steering operation, a torsion is induced in such bar to cause an angular phase difference between the steering shaft and the pinion shaft. In response to the angular phase difference so caused, the servo valve is actuated to control suction and exhaust of an actuating fluid for a power cylinder in such a manner as to generate an adequate assist force conforming with the angular phase difference. The servo valve functions to an assist force in proportion to a manual steering torque applied to the steering shaft, which causes the angular phase difference. In accordance with an increase of the angular phase difference, the assist force is rendered greater. In case the angular phase difference is decreased to the contrary, the assist force is rendered smaller. And supply of the actuating fluid to the servo valve is performed with the rotation of a pump driven by the motive power from an engine.
However, in any automobile where its engine is not installed in the front thereof (as in a midship car for example), the engine is considerably spaced apart from the steering shaft and so forth, and the use of a connecting pipe through the distance therebetween makes the structure complicated to eventually bring about some problems.
In view of such circumstances, there is contrived an improvement as disclosed in Japanese Patent laid-open No. 56 (1981)-99859, wherein an electric motor is disposed in the vicinity of a steering shaft, and a pump is rotated by the power from such motor to supply an actuating fluid.
In addition to the above, there is also known an electric power steering apparatus where a steering shaft or a pinion shaft is driven directly by means of an electric motor without utilizing any hydraulic action.
In each of such hydraulic and electric power steering apparatus, steering operations are performed frequently when an automobile is running at a low velocity or on any road with many curves such as a mountain road. Therefore, on any occasion requiring a high steering frequency, it follows that a load current comes to flow continuously in the electric motor, thereby inducing a possibility of overheating of the electric motor.